god help me (fuse)

hello everyone 
I hope you can help.
I have been using a friends radio and now it wont stop blowing the inline fuse on the power cable.
i replaced it a number of times .
to no avail.
it powers on for a phew minutes then pops.
any ideas
thanks

Das radio blowenfusen und corkenpoppen mit spitzensparken?


(Apologies, just light hearted humour for a bad situation. )


How many 'few minutes' before blowing the fuse? On receive or transmit? (Obviously transmit uses more power than receive.) Does the radio light up and work before blowing the fuse?
What happened previous to it blowing fuses? Is the power lead all OK?

A lot of questions but you've pretty much given us all very little to go on. I'm not surprised if it's a friends radio, you're bound to be panicking!

If it's just blowing on transmit it points to a few possibilities.

Are you sure that you've got the correct rated fuse in place? Fuses are easily misidentified and often mis-supplied especially the difference between quick blow and slow blow.

If the power supply is at fault it will lose its regulation on higher current demands if it's a linear rather than switched-mode type. This can cause fuses to blow, the voltage can rise as well as fall with linear regulation and that could make the radio momentarily draw more power as the power consumption increases. (The finals or output transistors will try to convert that extra voltage into more RF power increasing current draw.)

If you can try another power supply or hook up to a decent battery such as a car type to eliminate that.

If after that the radio still blows fuses then clearly the radio is at fault.


Sorry I can't be more helpful than that but diagnosing problems online from a distance can be fraught with problems.

I hope you get sorted out.


All the best,
Victor

Hi Victor.
first thanks for the help.
the power i have been using is a car batt.
i think the probelem must be internal.
its an old at555 that's been sat in a cupboard for a while.
i also noticed that it was off freq on ssb. but fine on the uk band.

Old radio.
Covers off big fuse watch for the smoke lol.
That was a test i did many times.
What do you say Victor.

That's certainly a 'fast & furious' method of fault finding Alan!

Fingers crossed the PCB tracks are a better 'rating' than the fuse.

Yep fixed many burned tracks lol.

Ha! Ha!

Check the reverse polarity diode. Usually they op just after you turn it on though.

If you have a multimeter, and know how to use it, measure the current (amps set for fuse rating) flowing into the rig on standby and quickly when you PTT. Then compare it with the manufacturers spec. Too high on one or the other would indicate a rig fault.

HOWEVER, It's not a good idea to leave it in line until the fuse blows as whatever is happening may be catastrophic enough to blow the multimeter fuse.

Second option. (field testing)
Covers off, run until the fuse blows again, and using your Mk.1 heat detector (your finger) find out what is glowing hot.

p.s. Have cool beer alongside you to quench your finger if burned.

Third thought.
If you are running it from a car battery, is the battery charger on the whole time?
If it is, and you have that multimeter, check that it isn't over-volting above 13.8 volts.
Especially when PTT.

Some cheaper battery chargers aren't exactly good at maintaining their voltage control when working hard and do 'surge' a bit.

Hope all that makes sense.
Good Luck.

Paul

well after a 2wk wait .i have radio back from shop.
apparently it was the inline fuse.(2 small)
yes i put a 2amp in.
so ok my mistake.still doesn't explain why the 10anp fuse blew .
i am currently waiting for a delivery of fuses as the shop never had any.
so I'm wondering how they can say it was just the fuse .
lets see what happens when i power it up.

lol
I did say big fuse watch for smoke.
Tested many like that years ago.
Hope that was the problem let us know when you get you fuses if it works.
Anybody tell us what fuse you have in yours ????.

hi all.
so my fuses arrived 2day.
and yes that was the probelem.
apparently these radios can draw 8amps.
so why do they come with a 2amp fuse installed.
thanks to all that helped trouble shoot my probelem.
i feel like a right pratt.

You didn't put the 2 amp fuse in so it's not your fault.

One reason I’ve never borrowed or lent a raisin from anyone!
If you don’t like the person then put a nail in it and deny all knowledge!